Measuring electrical signal amplitude changes



L A Y R R E T .lw v

MEASURING ELECTRICAL SIGNAL AMPLITUDE CHANGES Fired June 7, 1943 5 Shaetg-Sheet' 1 'lkllll lllll a 1947. V, J, TE RY E 2,41%07 MEASURING ELECTRICAL SIGNAL AMiLITUDE CHANGES Fild June 7, 1943 5 Sheets-Sheet 2 F/GZ.

TIME MILL/SECONDS $50 n so IOO 150 v 200 250 300 350 400 E i I l I 5 I A 1947- v. J. TERRY ETAL 2,419,607

MEASURING ELECTRICAL SIGNAL AMPLITUDE CHANGES Filed June 7. 1943 5 Sheets-Sheet 3 4 FIG.3

v INVENTORS: Vida? Jahn Terry and 1220mmmfem'ckq I April 29, 1947- v. J. TERRY ETAL 2,419,607

MEASURING ELECTRICAL SIGNAL AUPLITUDE CHANGES 5 Sheets-Sheet 4 INVENTOR8= Filed June 7, 1943 V I Vidal-John Tkrry, and Tfiamasi'rederick fizlcylhrgzaves v ATTORNEY;

t iuteuteri rtiprl MEASURING ELECTRICAL SKGNAL AMPLITUDE CHANGES ration of Delaware Application June '7, 1943, Serial No. 489,904 In Great Britain April 14, 19%

22 Elaims. l

The present invention relates to arrangements for measuring electrical voltage or amplitude changes, with particular application to blind landing systems for aircraft.

One of the well known methods for indicating the course for a landing aircraft is to transmit two similar overlapping, but difierently directed, radiation patterns from two corresponding aerials, on different wave lengths, the waves being respectively modulated by interlocking dot-dash signals. The aircraft receives the Waves with constant amplitude when on the right course, but when off the course it receives the dots with greater amplitude than the dashes on one side of the course, and with smaller amplitude than the dashes on the other side, the difference between the two amplitudes being a measure of the departure of the aircraft from the course. The signal used for modulating the waves may consist of an audio frequency tone switched on and ed in a series of dots and dashes, the on periods being dots for one side of the course and dashes for the other side. The signals may be detected and applied to ordinary head-receivers, or may be further rectified and used to operate a direct current meter, the readings of which can be made to indicate when the aircraft is on the desired course, or on which side thereof it is flying. Various other arrangements have also been used to give the required indications.

The present invention is directed principally is a change in the amplitude dlilerence, when it moves to the new reading.

The invention has, however, other applications besides to blind landing of aircraft. It could be used in any circumstances where a repeated change of amplitude or voltage of a signal is to be measured or applied to perform some function.

According to the invention there is provided an electrical arrangement in which a repeatedly occurring voltage change is caused to produce in a branch of a thermionic valve circuit a current determined in magnitude and sign by the said voltage change, by applying the voltage before and after any change respectively to charge two condensers, and by employing the difference between the charges acquired by the condensers to control the valve circuit.

According to another aspect, the invention provides an arrangement for indicating in magnitude and sign a change which repeatedly occurs in an electric voltage, in which two condensers. are respectively charged to potentials corresponding to the values of the voltage before and after a change, the difference between the charges of the condensers being then shared with a third condenser or group of condensers; and in which the potentlai acquired by the said third condenser or group of condensers is applied to a thermionic valve circuit in such a manner that a current measuring instrument included in the towards improving the operation of a meter indlcation system. In some of the existing arrangements, the average reading of the meter is pro portional to the difference between the dot and the dash amplitude, but a large proportion of the current may flow through the meter during 1 In order that the the short period of the dot. meter needle may reach the required reading in a reasonable time, it must have a short enough natural period to follow the impulses fairly closely, and is therefore subject to kicking, which makes it difilcult to read. The present invention proposes arrangements whereby'measurements are made of the clflerence between the dot and dash amplitudes at suitable times. The result of the measurements is applied to the meter so that the needle rapidly reaches the required reading and remains practically steady (there being a slight decay). The amplitudes are periodically stored in condensers so that after the needle has reached the final indication, the. meter is no longer subjected to any appreciable impulses, and

therefore the needle remains steady until there valve circuit indlcates'the value of the said voltage change.

According to a further view, the invention may consist in an arrangement for deriving an electric current which depends in magnitude and sign upon the change in amplitude of an electric signal wave, which amplitude repeatedly alternates between two values, comprising means for charging two condensers. respectively to potentials corresponding to the said two values, means for applying the diderence of the said potentials to charge a third condenser or group or condensers, and

means to apply the potential acquired by the third condenser or group of condensers to control a thermionic valve circuit in a branch of which the said substantially constant electric current is generated.

According to still another aspect, the invention comprises an arrangement for obtaining a steady reading on a current measuring instrument connected in a branch of a thermionic valve circuit, which reading indicates the magnitude and sign of a constantly repeated change in amplitude of an electric signal wave, comprising means for 3 deriving from the wave two electric voltages proportional respectively to the amplitudes of the wave before and alter a change, means for applying the said voltages respectively to two condensers, means for connecting the two condensers to a third condenser or group of condensers so that the combination acquires a charge proportional to the difference between the charges of the two first mentioned condensers, and means for applying the potential acquired by the combination to control the valve circuit. v The invention will be described with reference to the figures of the accompanying drawings.

Figs. 1, 2, 5 and 6 show diagrams used for explaining the invention, and Figs. 3 and 4 show schematic circuit diagrams of two embodiments. In order to explain the invention clearly its application to a particular blind landing system will be described; but it is to be understood that it is not restricted to this application. Referring to Figure 1, sketches (1) and (2) show the form of the voltage of the signals which will be received by the aircraft on one side of the course, and on the other side respectively. They consist of trains of audio frequency waves, for example, at 1000 cycles, the amplitude of which varies according to the dot-dash signals as shown by the outlines. Actually the outlines of the dots will not be absolutely square as shown, but will be somewhat modified in the neighbourhood of the leading and trailing edges on account of the filters or tuned circuits in the transmitting and receiving apparatus. According to the invention, the voltage change 1: which takes place on the occurrence of the dot is determined both in sign and magnitude by measuring the voltage during two short periods hi to t: and is to t4 chosen during and after the dot at times when the wave form is not appreciably affected by the filters. If one embodiment of the invention, the voltage 1) is used to charge a condenser connected to the control grid of a valve having a direct current milliammeter in the plate circuit. Thus if the voltage a remains-substantially thesame for a succession of dots, the potential applied to the grid will be steady (except for a slight leak which will be referred to later on), and the meter reading will therefore also be steady, since the dots do not have any separate eflect on the meter.

If the voltage 12 should change, the grid voltage will change accordingly and the motor will take assume a potential proportional to Ul-7)2, that is,

proportional to v. The proportion depends on the capacitiesof the three condensers, and in order to avoid reducing 12 too much, K: and K: should be large compared with K4. The potential of K4 will increase at each subsequent operation until it becomes substantially equal to v,

v and the time taken for this to occur decreases up the new reading directly. Although an interdensers charged in series from the'voltage 0 so that these potentials change in opposite directions. I

Figure 3 shows a schematic circuit diagram of one form of a receiving arrangement for-blind landing according to the invention. The signals.

such as those shown in (1) or (2) of Fi re 1, are

as the capacity of K4 is decreased. The valve V1 is connected as a cathode follower, with the resistance R5 connected in series with the cathode in the well known way. A potentiometer comprising the resistances R-z, P, and Re is connected across the high tension supply HT. The condenser K4 is connected to a point on this potentiometer as shown, so that the "control grid may be operated at a suitable point on the valve characteristic. A centre zero milliammeter M is connected through a resistance Re to an adjustable point on the resistance P, the adjustment being made so that the milliammeter reads zero when no signals are being received. The milliammeter M is calibrated to indicate the direction and amplitude of the voltage change, thereby lndicating the side of the beacon path at which the apparatus is located, and the distance from the signal source. A resistance R3 is connected across the condenser K4 and is provided for the purpose of slowly dischargin the condenser K4 so that the meter reading ultimately returns to zero when there are no incoming signals. The time constant of the circuit K4, Ra should however be high enough so that the condenser does not have time to discharge appreciably between the successive dots of the incoming signals.

The condensers K2 and Ka should be equal and as already stated should preferably be large compared with K4, so that connecting the latter does not reduce the voltage 1) very much. For example, K2 and K: could be 1 microfarad each,

and K4 could be 0.1 microiarad.

The sequence of connections of the condensers K2, K3 and K4 is carried out by the train oi five relays A, B, C, D and E. This train is set in operation by the dot signals shown in Fig. l (3) ing to the dots and dashes. A low pass filter L, K1 is provided for smoothing the output 01 W1 so that the wave form of the voltage obtained across the resistance R1 will be substantially as shown in Figure 1, (3) or (4). With the help of a train of live relays A, B, C, D and E (whose operation will be explained in detail below) the or (4) which are obtained across the resistance R; as already explained. The'voltage across R1 is applied to the series circuit comprising the two condensers K5 and KB and the diagonal points of a second bridge rectifier W2. As far as the condenser circuit is concerned, this-rectifier together with R4 may be considered as equivalent to a serially connected resistance, and when a dot similar to' Figure l (3) arrives, the form of the voltage across this equivalent resistance will be as shown in Figure 1 (5), which is seen to be two oppositely directed pulses whose form depends on the time constant of the series circuit. If the dot had been similar to Figure 1 (4) the two pulses would both be inverted.

The action of the rectifiers is however such that the voltage pulses obtained across the other diagonal points of the bridge will be in the same direction, as shown in Figure 1 (6), irrespective of which kind of dot was received. These voltage pulses areapplied to the control grid'oi a. valve V: through a resistance R4 shunted by a conenser K1, the grid being blessed to cut-ofi irough a resistance R2 as shown. The valve Va as connected in series with its plate circuit a inding of the first relay A, theoperation of *hich starts a cycle of operation of the relay The resistance R4 and condenser K1 are proided because the magnitude of the pulses aplied through V2 to the relay A may vary rather 'idely. These elements are for the purpose of reventing a large pulse from holding up the clay A too long, so that it interferes with the peration of the relay train. When the pulse a large, grid current flows and charges the con- ,enser K1 so that the grid tends to assume a .egatlve potential. When the pulse potential has 'assed the maximum value and has fallen below he potential to which K7 is charged, the valve '2 is cut off, thereby allowing A to be released.

The relays are shown according to the detached ontact principle. Each relay winding is desigrated by a capital letter, and its sets of contacts vy the corresponding small letter, with a numeral distinguish the different sets on the same relay. ill relays are shown in the unoperated condition, which. exists before the relay battery RB has won switched on by throwing in switch SW. Reays B and D are slow to release, and relay C slow to release and also slow to operate. As- :uming that no signals are being received, when iWl'GCh SW for relay battery RB is switched on, clay B operates; closing of contacts b3 operates 3; closing of contacts 52 does not operate relay .4. but only prepares a locking circuit for A hrough its right hand winding which is insufii- :ient to pick up the relay armature but sufficient ;0 hold it up, once it has been picked up by the .eit hand winding of relay A; closing of con- ;acts in prepares a charging circuit for condenser Biz. The operation of D opens the contacts (11- and closesdz to prepare a circuit for the operation of C. Thus when the circuit is ready to nection orthose condensers to one end of the receive signals, relays B and D are operated,

contacts b1, b2, b3 and C12. are closed, and 111 are open. 1

The subsequent operations of the relay train will be explained by reference to Figure 2.

At the top of Figure 2 is a time scale divided into milliseconds, to which all the diagrams are referred. The curve S represents a dot, similar for example to Figure 1, (3) but with the leadresistance Ri'thr'ough its contacts ea. The operation of C is delayed until about milliseconds, when it completes 'the charging circuit for K: by connecting it to the other end of R1 through the contacts 01. Referring again to Figure 1, this occurs at the time h which is the beginning of the first measuring interval. The operation of C disconnects the winding of B by changing the contacts 02 from top to bottom position. Thereby E and C remain energised in parallel to battery RB even after relay A has subsequently been released. The release of B is delayed by about 60 milliseconds; through contacts b2 the right hand winding locking circuit is operated (so that A releases at about 120 milliseconds). At the same time b1 is opened and the condenser K2 is disconnected, which will have been charged up from the voltage m (Figure 1, (3) or (4)). The release of B occurs at the end of the first measuring interval (time t2 in Fig. 1). When the second of the pulses shown in Figure 1, (6) arrives at about 125 milliseconds, it momentarily operates A, but this has no eflect, since A cannot now lock up.

The release of B also disconnects D by opening'the contacts in at about 110 milliseconds. The release of D is delayed until about 300 milliseconds, which is the commencement is or the second measuring interval. At this time, contacts (11 close and complete the charging circuit for condenser Ks (which charges up to the voltage or) and contacts (is open to disconnect relay C, which releases at about 360 milliseconds. This opens contacts or, so disconnecting K1 and marking the end t4 of the second measuring interval. Contacts'cz change back, disconnecting E and energising B. Relay E releases at about 380 milliseconds and connects condenser Ki to the condensers K: and K3 by changing over its contacts 61 and er. K4 then charges up to a voltage proportional to var-v1, that is, proportional to v, as

' already explained. Relay B operates at about ing and trailing edges shown as they might be modified by the filters in the circuit. Below the curve S are shown a number of horizontal lines marked As to E and their lengths represent the times during which the corresponding relays are operated. The initial dotted portion of each denotes the period during which the relay is energlsed but has not operated. The designation 1' indicates the point at which the operating current is cut off from the corresponding relay, and the termination of the horizontal line denotes when it releases. The set of lines A2 to E2 are those which apply when the relays have maximum releasing times. Below this set is shown another set A1 to E1 which apply when the relays have minimum releasing times, and. this gives an idea of the latitude allowable, and shows that I these times are not critical.

When the first of the pulses. shown in Figure 1,

,(6) is applied to the grid of valve V2, it operates lrelay A, which looks up by its own contacts or through the circuit prepared .by contacts in. The closing of the'contacts a1 energizes C through the closed contacts iii, and also energises E 390 milliseconds and the circuit is back in its original condition, The process is repeated each time a dot is received, so that when the amplitude of the dots remains constant, condenser K4 will be maintained at a substantially constant potential (except for the slight leak through R3 between the dots). When the signals cease, the charge on K; will leak away through R: sov that its potential reaches approximately zero in a reasonable time (a few seconds, for example).

It will be seen that the timing of the relays has been chosen so that the first measuring interval M's indicated near the curve S occurs as late as possible without at the same time encroaching on that part of the wave form afiected by the filters. The corresponding second interval Na is rather late, and well away from the dot. 1

The interval M2 is determlned'by the operation time of relay C and by the release time ofrelay B. and N: by the release time of relay D and by the release time of relay C. The group of lines A1. to E1 shown in Figure 2 indicate a different timing by which both the measuring intervals M1 and N1 occur as early as possible while avoidmg the wave form distortion. The timings of lays B, O andD have all been made shorter. it will be apparent that any intermediate arrangement will operate satisfactorily, so that no precise adjustments of the relay timings are required. It will, of course, be necessary to ensure that the shorter intervals M1 and N1 are sufliclent to allow the substantially complete charging of condensers K2 and m.

Fig. 4 shows another embodiment of the invention, which is the preferred form. This operates according to the same principles as Fig. 3, but includes certain other features by which its performance may be improved. The measuring functions are carried out by a relay train which is difierently arranged and is started in a different manner.

The circuit of Fig. 4 comprises four thermionic valves which have been shown as triodes for simplicity, but valves having any number of grids appropriately arranged and polarised may be used if desired. Of these, the valves V6 and V6 serve a double purpose. They are used in parallel to serve as an initial amplifying stage for the signals which are applied through the transformer T1. They are also used as part of the measuring bridge circuit to determine the difference of potential of the measuring condensers,

and with the meter M and associated resistances carry out the same function as the valve V1 of Fi 3.

Valve Vs has substantially the same function as V: in Fig. 3 and V1 forms an amplifying stage for V8. 1

The two valves V5 and VB have their plates connected in parallel through the primary winding of the output transformer T2 to the positive terminal of the high tension source HT. The two control grids are connected through corresponding series resistances R11, R13 and R12, R14 in parallel to one terminal of the secondary winding of the input transformer Tl, the other terminal of this winding being connected to the oathodes through the bypass condensers K15 and Kit. The resistances R11 and R111 are equal, as also are R15 and R14. The cathodes are biassed positively by the cathode current potential drops in the equal resistances R15 and Ru: and the corresponding portions of the potentiometer P1, and the control grids are given a suitably lower positive potential by means of the potentiometer comprising resistances R18 and R19 connected across the circuit comprising the resistance R20 and the condensers Km and Kw, which circuit corresponds 'in function to the filter L, K1 of Fig. 3, and is a cheaper arrangement. The load resistance R21 corresponds to R1 in Fig. 3, and substantially the same type of voltage wave will be obtained across it, as shown in Fig. 1 at (3) or (4).

These voltage Waves are used to charge in turn the two equal condensers K11 and Kai (corresponding to K: and K: in Fig. 3) by the action of the relay train which will be presently described, and the potential difference of the two condensers is then used to charge the two equal condensers K1: and K14 in series, which (.01 densers together correspond to K4 of Fig. 3. W111 this occurs, the potential of the control grid one of the valves V5 and V6 will be raised, ax that of the other will be lowered by an equ amount. Thus the plate current of the one val will be increased, and that of the other will 1 decreased, so that the two cathodes will acqui diflerent potentials, the difference being mea ured by the rnilliammeter M connected in seri with a suitable resistance R17 between the ti cathodes. If the initial control grid potentia are so chosen that the valves are operated 1 the straight part of the characteristic curve, thi the meter readin will be substantially propo tional to the difference or the potentials oi t1 two condensers Kn and K12. As in the case .Fig. 3, the potentiometer P1 will be adjusted obtain a zero reading on the milliameter M Whi no signals are being received.

It is to be noted that the amplifying action the valves will be substantially unaffected by t] measuring function, since the total plate currel flowing through the transformer T2 remains col stant.

As in the case of Fig. 3, condensers K11 and B should preferably be large compared with K1: 9.! K14; and the resistances R13 and R14 should 1 large enough to prevent appreciable leakage the charges of K13 and K14 between successi' charging operations.

The voltage across resistance R21 is also appli:

to the valve V1 through a condenser K19 whit corresponds to K5 and Ka of Fig. 3, the shunt r sistance R23 corresponding to R2. Thus puls like Fig. 1, (5) will be applied to the control gr of V1; there is however in this case no rectifi corresponding to W: of Fig. 3 for a reason to I explained later. The pulses are applied throu! a resistance Res to the control grid of V7 whiz is blessed from the cathode by resistance R21.

rectifier W5 is connected between the control gr and cathode and is directed so as substantially shortcircuit the control grid and cathode it tl amplitude of the signal should be such as to tax to make the grid positive. Rectifier W1 is lik wise provided to prevent the potential of the CO1 trol grid from becoming excessively negative as to risk'blocklng the valve, and the rectlfl is suitably biased with respect to the cathode 1 means of the two resistances R21 and R29, tl extra resistance being used to provide the a1 ditional bias necessary to allow an appropria range of variation for the control grid potentii The plate of V715 connected to the high tensir source through the resistance R31 and to the col trol-grid oi the valve Va through thecondens K20 and resistance R28. The valve Va is equippi in exactly the same way as V1, the elements R4 R26, Rat, Rat. W4 and We, corresponding respei tively R23, Rea, R17, R29, W5 and W1. The pic. current of Va is'however supplied through a inductance In instead of through a resistanc and the plate is connected to the starting rele F through a'condenser Kn.

It is to be noted that the limiting rectifiers a: necessary for both valves because the range 1 amplitude of the incoming signals is liable to 1 large. When the signals are weak considerab amplification is necessary, and none of the rei tifiers will operate. For strong signals, althoug W5 and W1 would operate, the maximum allov able output of V7 would still be able to block Va, i that W4 and We are required to prevent this.

The resistances Ru and R12 are provided 1 anaoov prevent appreciable radio frequency potentials workin of the circuit. Resistances R25 and Rec are provided to limit the grid currents of the corresponding valves so that the charging and discharging of the condensers Km and K20 will not be appreciably afiected.

The relay F should be a non-polarized or neutral relay, preferably relatively quick to operate. The inductance L2 and condenser K21 act as a high pass filter, so that the wave shown in Fi 1 which is applied to V-: will appear substantially as shown in Fig. 5 when applied to the relay l This consists practically of two relatively narrow pulses both of which will momentarily operate the relay, since the latter is non-polarized. Thus it does not matter whether the first of ese pulses is upwards or downwards, and it is store not necessary to provide a rectifier havi the function of W: in Fig. 3 designed to turn pulses all the same way. It will be clear from what has been said that the rectifier-s W4 and l/l do not correspond to We, out they pera function similar it; of Fig. 3. The relay F corresponds to the relay ll. oi Fig. 3, audio used for starting the relay train G, H, J, Q, which carry out the same function as the train to that of the network iii, Qlil, E, but in a difierent way. Relays H, J

iii are slow release relays. I lvhen the relay battery RE is first switched Q operates through the contactsgi, operates through contacts qsand is. ya b are both already closed. Relay Q also precircuit for relay G .by closing acts c2. Relay H closes contacts In; in parallel with go, and operates relay J by closing contacts he.

contacts 1; thus operating relay G, which locks itself through the contacts Q3. The second operation of F at the end of the dot will thus have effect. Relay .3 being operated, relay H is disconnected by the contacts go but does not ree for 2D to 40 milliseconds, and maintains for this time the current energising relay J, which current passes through the contacts in. When.

releases, J remains operated ior 2G to 4:0 milliseconds and then releases, disconnecting relay Q and re-operating Hr. Relay Q is designed to hold up for roe to 200 milliseconds and onlts release at the end of that period it opens contacts qr, but relay G does not release since it is held operated the contacts in. The opening'of contacts Q's releases. ll after 20 to 40 milliseconds and this in turn releases G by opening the contacts ha. Vfhen releases, Q, H and J are re-operated by the his. o shows a timing diagram of the same i type as Fig. 2 and applicable to the relay train oi his. The lines F1 to Q1 representthe operato tlons of the relay F to Q when the release time of H and J is the minimum 20 milliseconds and that of Q is the minimum 100 milliseconds, the corresponding measuring periods being M1 and N1. The lines F2 to Q: are for the maximum release times of 40 and 200 milliseconds, respectively, the corresponding measuring periods being Ma and N2.

Referring to the measuring circuit of Fig. is

and also to Fig. 6, just before zero time, contacts 11 are closed, qr are open, or and or are as shown (relay G not yet being operated) and contacts hr are changed over. Soon after the relay F operates, G operates and changes over the contacts or and 92, thus preparing the condensers K11 and K12 to be charged. When relay H releases at: time h, the condenser K11 will be connected .1

across R21 since contact 71 is closed, and will be.

charged up to the voltage corresponding to they dot period. When relay J releases at is, con:

tacts 11 open, disconnecting K11. Shortly afterwards relay H is again operated and prepares condenser K12 for charging, which occurs at ts when relay Q releases, closing the contacts (11. Condenser K12 is disconnected at t4 when H is again released. Condensers K11 and K1: are now .charged up respectively to the voltages corresponding to the dot and dash periods. The release oi G, which occurs very shortly afterwards, connects K11 and Km to K13 and Km by changing back the contacts or and g2, and the voltage difference is then indicated on the milllammeter M.

It should be understood that the embodiments described with reference to Figs. 3 and a are only two possible forms or the invention applicable to blind landing of aircraft, and the invention relay F is momentarily operated and closes the can be employed in other fields: for example wherever a repeated voltage change occurs or can be derived, this change can be measured according to theprinciples explained or can be made to carry out some function by replacing the meter M by a relay. or other utilization device. A particular case arises in telegraph sys terns where signals are conveyed over a channel by the'alternation of two circuit conditions, as in single or double current systems, or in carrier current systems from which single or double current signals can be derived. The voltage changes which represent the signals in such systems can be measured by the method of the invention or can be made to operate apparatus if desired. The essential feature of the invention is the production of a substantially constant current in the branch containing the meter M, which current is proportional to the voltage change 1;.

It will be evident to those skilled in the art that the relay trains illustrated in Figs. 3 and 4 represent only two possible ways of arranging the relays to perform the desired sequence of operations. Alternative circuits employing other series of relays, or sequence switches or the like would clearly also be possible.

It is further to be noted that while in many cases the voltage change 2; is periodically or regularly repeated, this is not essential, The voltage changecould be quite irregularly repeated and a substantially steady reading on the meter M would beobtalned so long as the leak of the condenser K4 in Fig. 3 (or condensers K13 and Km in Fig. 4) were chosen with regard to the longest times between successive repetitions.

Thus good measurements could be obtained dur-- ing the passage of a general telegraph signal in the Morse code or any other code.

Thus in the claims the term repeated" or "repeatedly" does not necessarily imply periodic or regular repetition at some definite frequency.

What is claimed is:

1. Apparatus for determining impulse voltage changes occurring in recurring cycles of impulses, comprising two capacitances, means for charging the first capacitance in proportion to an impulse voltage in a cycle of impulses before the change, means for charging the second capacitance in proportion to an impulse voltage in said cycle of impulses after the change, means for determining a substantially constant voltage representing the difference between the charges on said capacitances, and relay means for automatically actuating said capacitance-charging and determining means during successive cycles.

2. Apparatus as set forth in claim 1 in which the capacitance-charging means comprises a relay system including relay-operated switch means for connecting the capacitances successively to a source of said voltages before and after the change, and thereafter to the determining means.

3. Apparatus as set forth in claim 1 in which the determining means comprises a resultant capacitance and means for applying to the resultant capacitance a charge proportionate to the resultant of the charges on the first and second capacitances.

4. Apparatus as set forth in claim 1 including means responsive to a voltage change in each cycle for actuating the first capacitance-charging means and for operating thereafter the second capacitance-charging means and the determining means whenever said first means is actuated.

5. Apparatus as set forth in claim 1 comprising means for actuating the first capacitancecharging means responsive to a voltage change in each cycle, and means for actuating the second capacitance-charging means after a predetermined interval succeeding the a'ctuation or the first capacitance-charging means.

6. Apparatus as set forth in claim 1 comprising means for deriving a voltage pulse from the initial voltage change in a cycle, and means responsive to said voltage pulse for actuating the first capacitance-charging means.

'7. Apparatus for determining changes in voltage from a source of electrical impulses comprising two capacitances, a relay system including first relay means for charging the first capacitance from the voltage of an impulse before the voltage change, second relay means for charging the second capacitance from the voltage of anim 7 relay means after a predetermined interval following the operation of the first relay means, and means including a delayed operation relay for actuating the third relay means after a predetermined interval following the operation of the second relay means.

9. Apparatus as set forth in claim 7 comprising means responsive to a change in voltage from said source for starting the operation of the relay system.

10. Apparatus for detecting impulse voltage changes in recurring cycles of impulses each including a plurality of voltage changes which comprises normally disconnected means for determining the impulse voltage before and after a voltage change in each cycle, and means responsive to a preceding voltage change in the same cycle for connecting and actuating the determining means.

11. Apparatus as set forth in claim 10 in which the actuating means includes means for producing a pulse actuated by said preceding voltage change.

12. Apparatus as set forth in claim 10 in which the actuating means includes means for gencrating a pulse actuated by said preceding voltage change, said pulse being generated in the same sense regardless of the direction of said preceding voltage change.

13. Apparatus for determining voltage changes in recurring cycles of impulses comprising means for determining the voltage at a predetermined initial point in each cycle, means for determining voltage at a predetermined later point in said cycle, said first voltage-determining means being responsive to a preceding impulse voltage change in each cycle.

14. Apparatus as set forth in claim 13 including means for actuating the first voltage-determining means after a predetermined interval following a preceding voltage change in each cycle and means for actuating the second voltage-determining means after a predetermined interval following the actuation of the first voltage-determining means.

15. Apparatus selectively responsive to signals which consist of a series of either dot elements or of dash elements produced by voltage changes, comprising first circuit means responsive only to the signal voltage occurring a predetermined period after the beginning of an element less than the duration of a dot element, second circuit means responsive only to the signal voltage occurring a predetermined period after the beginning of said element greater than the duration of a dot element and less than the duration of a dash element, and comparative circuit means responsive to the resultant of said signal voltages to measure the difference thereof.

16. Apparatus as set forth in claim 15 in which each means responsive to a signal voltage comprises a capacitance charged by said voltage, and the means responsive to the resultant of said voltages comprises a capacitance charged from said capacitances connected in series.

17. Apparatus for automatically determining position relative to the course indicated by a radio beacon of the type which produces a steady voltage signal along said course, dot signals at one side of the course and dash signals at the other side of the course, comprising first circuit means responsive only to the signal voltage occurring a predetermined period after the beginning of a signal less than the duration of a dot, second circuit means responsive only to the signal voltage occurring a predetermined period after the beginning of said signal greater than the duration of the dot and less than the duration of a dash, and comparative circuit means responsive to the resultant of said signal voltages to measure the difference thereof.

it, detecting aatus iii no ifoi" a reeui iii;

2 the voltages of impulses before and after the :hange, said means comprising a pair of thernionic valves connected in parallel to receive and .mplii'v said impulses, means tor producing a curentwproportional to said resultant voltage, said astmentioned means including the cathode cirmts of said two thermionic valves and means or impressing said resultant voltage between he control grids of said valves.

19. Apparatus as set forth in claim 18 in which he valve system includes an indicating device esponsive to said proportional current, and means for adjusting said current to zero when no nput voltage is applied to the system.

20. Apparatus as set forth in claim 13 in which :athode follower includes two valves having :athode follower connections, and in which the iroportional current is generated. between the :athodes.

21. Impulse voltage change determining apiaratus comprising two capacitancea'means for :harging the first capacitance with an impulse 'oitage in proportion to an impulse voltage be ore the change, means for charging the second :apacitance in proportion to an impulse voltage after the change, a third capacitance, and means 'or applying the charges of said first and second :apacitances differentially to said third capaciance thereby producing a resultant of said first :apacitance and said second capacitance in said .hird capacitance, the output of which is a sub- :tantially constant voltage representing the iifierence between charges on said first and sec- )nd capacitances.

22. Impulse voltage change determining apiaratus comprising two capacitances, means for :harging the first capacitance with an impulse foliage in proportion to an impulse voltage be- Fore the change, means for charging the second :apacitance in proportion to an impulse voltage after the change, and determining means com- :rising a thermionic valve having input and output circuits, input circuit further comprising a thim capacitance, a means for applying to said third capacitance a substantially constant voltage proportional to the resultant of the voltages 0! said first and second capacitances, and indicating means responsive to current flow in the output circuit.

VICTOR; JOHN TERRY. THOMAS FREDERICK STANLEY HARGREAVES.

REFERENCES CITED The following references are of record in the file of this patent:

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